Method of manufacturing carbon fibres

ABSTRACT

The present invention relates to a method of manufacturing carbon fibres from raw materials of renewable origin, comprising:
         a) synthesis of acrolein from glycerol of vegetable origin;   b) ammoxidation of the acrolein to obtain acrylonitrile;   c) polymerization of the acrylonitrile to a homopolymer or copolymer of acrylonitrile (PAN);   d) conversion of the PAN to PAN fibres;   e) partial oxidation of the PAN fibres; and   f) carbonization of the partially oxidized PAN fibres. It also relates to the fibres capable of being obtained according to this method, and also to the uses thereof.

Method of manufacturing carbon fibres The present invention relates to a method of manufacturing carbon fibres from raw materials of renewable origin, and also to fibres capable of being obtained according to this method.

Carbon fibres are materials composed of very fine fibres having a diameter of 5 to 10 microns, for which carbon is the main chemical element. Other atoms are generally present such as oxygen, nitrogen, hydrogen and less often sulphur. The carbon atoms are bonded together and form crystals of graphite type that are more or less aligned parallel to the axis of the fibre. Several thousand of these fibres are twisted together to form a strand. These strands may be used alone or made into the form of a fabric.

Carbon fibres are one of the materials that are developing fastest at the current time. Used as reinforcements for thermosets such as crosslinked polyester resins and epoxy resins, or as agents for improving certain thermoplastics such as polyamides, they actually make it possible to obtain composites that are very strong relative to their weight and even that are often stronger than steel per unit weight.

These composites thus have very good tensile moduli, very high tensile strength and a low thermal expansion coefficient. They can replace metals in many applications, for example aircraft or spacecraft parts, in sports equipment (tennis rackets and golf clubs) and in the structural resins used in wind turbines. Carbon fibres also find applications in the filtration of high-temperature gases, as antistatic agents, as specialty electrodes due to their corrosion resistance, as reinforcement for pressurized gas tanks, especially for storing hydrogen.

The origin of carbon fibres dates back to 1958, the date when they were proposed by Union Carbide. The first fibres were of mediocre quality but rapid improvements were obtained using polyacrylonitrile (PAN) as a precursor. Depending on the precursor, the fibres are not identical: those obtained from PAN are fairly turbostratic, with graphitic planes that have a non-zero angle with one another and which are randomly folded. The fibres obtained from pitch are graphitic after heat treatment at more than 2200° C. The fibres obtained from these various precursors therefore also have different properties. Thus, those obtained from PAN have, in general, a better tensile strength.

The synthesis of carbon fibres from PAN typically comprises a step of firing the fibre at about 300° C., in order to oxidize it slightly, after which the fibre is placed in an inert atmosphere under argon or nitrogen at a temperature of around 2000° C. in an electric furnace. After this step, the fibre contains around 95% carbon. A subsequent treatment at a higher temperature increases its thermal conductivity and its modulus.

It is therefore understood that carbon fibres are very energy-intensive products, due to the high temperatures necessary during their fabrication. This energy consumption is not compensated for by their ability to allow the manufacture of lighter vehicles and to consequently reduce the energy consumption in transport applications.

In the countries where the electricity production comes predominantly from the consumption of fossil resources, this energy expenditure is accompanied by an increase in the emission of greenhouse gases, which are particularly harmful for the environment. In order to reduce this emission, it is not easy to act on the method of synthesizing carbon fibres. On the other hand, it seems possible to use raw materials that have a lower environmental impact than the PAN conventionally used in the synthesis of carbon fibres, in particular renewable or biobased raw materials.

In this context, one subject of the present invention is a novel method of synthesizing carbon fibres from raw materials of renewable or biobased origin.

More specifically, one subject of the invention is a method of manufacturing carbon fibres, comprising:

a) synthesis of acrolein from glycerol of vegetable origin;

b) ammoxidation of the acrolein to obtain acrylonitrile;

c) polymerization of the acrylonitrile to a homopolymer or copolymer of acrylonitrile (PAN);

d) conversion of the PAN to PAN fibres;

e) partial oxidation of the PAN fibres; and

f) carbonization of the partially oxidized PAN fibres.

This method will now be described in greater detail. It should be noted, by way of introduction, that it may comprise steps other than those mentioned above and in particular one or more steps prior to step (a), one or more steps after step (f) and/or one or more intermediate steps, as long as these steps do not negatively affect the entire method, in particular the yield and/or the quality of the carbon fibres obtained.

In the first step of the method according to the invention, acrolein is formed from glycerol of vegetable origin.

According to one preferred embodiment of the invention, the glycerol used in step (a) is obtained as a by-product of a transesterification of triglycerides of vegetable origin. A conventional transesterification reaction actually uses a linear C₁-C₁₀ monoalcohol, such as methanol or ethanol, or a cyclic C₃-C₆ monoalcohol, which is reacted with triglycerides to obtain alkyl esters of C₁-C₁₀ alcohols and glycerol. Triglycerides are compounds of formula: R₁—CO—O—CH₂—CH(OCO—R₂)—CH₂—O—CO—R₃, in which R₁ to R₃ denote saturated or (poly)unsaturated, linear or branched C₁₀-C₃₀, for example C₁₂-C₁₈, alkyl groups, which are an important constituent of vegetable oils and fats such as palm oil, linseed oil, groundnut oil, coconut oil, sunflower oil, soybean oil or rapeseed oil. The latter is especially transesterified in the manufacture of biodiesel recommended as a replacement fuel for fossil fuels. The glycerol used according to the invention may therefore be a by-product of the manufacture of biodiesel from vegetable oil. By way of indication, around 1 tonne of glycerol is obtained from 10 tonnes of fatty acid triglycerides.

The transesterification step is generally carried out at a temperature of 20 to 150° C., preferably from 25 to 100° C., more preferably from 25 to 80° C., for example over a period of 4 to 8 hours, in the presence of acid or basic catalysts, preferably in the presence of a basic catalyst such as sodium or potassium methoxide, in a solvent such as methanol, in a stirred reactor (in particular under high shear) or a fixed or fluidized bed reactor. As a variant, the transesterification may be carried out in the presence of supercritical methanol at high temperature and pressure. In general, all the reactants are dehydrated to prevent the saponification of the triglycerides and to enable an easier separation of the glycerol.

In step (a) of the method according to the invention, the glycerol is dehydrated to acrolein in the liquid phase or in the gas phase. According to one preferred embodiment of the invention, the dehydration of the glycerol to acrolein is carried out at 250-350° C. and 1 to 5 bar, in the presence of molecular oxygen and advantageously with an acidic solid catalyst, as described in Application WO 2006/087083, preferably in the presence of a catalyst of strong acid type having a Hammett acidity function H₀ of between −9 and −18, as described in Application WO 2006/087084.

As a variant, the dehydration of glycerol may be carried out as described in Application US 2008/119663, by heating at a temperature ranging from 250 to 400° C., preferably from 260 to 300° C. In the case of a reaction in the liquid phase, the pressure is adjusted, for example between 1 and 50 bar, so as to keep the reaction medium in the liquid state. A homogeneous or heterogeneous acid catalyst, and/or salts of mineral acids, such as potassium or sodium (hydrogen)sulphates, may be used to accelerate the reaction. It is thus possible to use a homogeneous acid catalyst such as sulphuric acid, phosphoric acid, toluenesulphonic acid or methanesulphonic acid or a heterogeneous catalyst such as a zeolite of HZSM-5 or MCM-22 type, metal oxides, such as aluminium oxide, covered by an inorganic acid such as phosphoric acid, or an ion exchange resin. As a variant, it is possible to use biocatalysts such as lipases or esterases.

The present description does not exclude the glycerol from being dehydrated to acrolein at the same time as it is formed from triglycerides, in the presence of a dehydration catalyst as described above, as taught in document US 2008/0119663. It will then be advisable, in this case, to then carry out a separation of the acrolein from the reaction medium by any suitable means, for example by distillation, extraction, phase separation or membrane separation. It is preferred however that the transesterification and dehydration steps are carried out separately, in order to optimize the yield of acrolein. In order to do this, the glycerol is advantageously extracted from the transesterification reaction medium by distillation, membrane separation or phase separation. It may optionally then be purified to remove the soaps, salts, and bases that it contains in a small amount, before being converted to acrolein.

The acrolein produced in step (a) of the method according to the invention is then subjected, in step (b), to an ammoxidation in order to obtain acrylonitrile, according to the following reaction scheme:

CH₂═CH—CHO+NH₃+0.5

CH₂═CH—CN+2H₂O

This ammoxidation step is well known to a person skilled in the art and may in particular be carried out at 200-450° C. by passing a mixture of acrolein, ammonia, air and inert gas over a catalyst formed from one or more oxygenated salts of arsenic and of less electronegative elements. A method of this type is described in Application FR 1 410 967. As a variant, it may be carried out as described in document DE-1 070 170, by using a molybdenum-based catalyst, at a temperature of 250-350° C., or as described in document U.S. Pat. No. 3,094,552, by passing the reaction mixture over a catalyst based on tin and antimony, at a temperature of 300-550° C., or else as described in Patent GB-709 337, in the presence of a catalyst formed from a mixture of silica, molybdenum oxide and phosphoric acid, at a temperature of 250-600° C.

The acrylonitrile thus obtained is polymerized, in step (c) of the method according to the invention. The acrylonitrile may be homopolymerized or, according to a preferred embodiment of the invention, copolymerized with at least one other monomer, preferably an acrylic monomer, that is to say a monomer of (meth)acrylic acid or of an alkyl ester of (meth)acrylic acid, such as methyl acrylate, methyl methacrylate or acrylic acid. Indeed, as long as it does not represent more than 10% by weight relative to the total weight of the monomers to be polymerized, this comonomer allows a better control of the thermal effects in the subsequent step (e) (Gupta, A. K. et al., JMS-Rev. Macromol. Chem. Phys. C31, 1991). This comonomer may itself be of renewable origin. Thus, the acrylic acid may be obtained from glycerol and the methyl methacrylate may incorporate, via its synthesis, acetone and methanol of renewable origin. Methyl acrylate is preferred for use in the present invention, since it is very close in polarity to acrylonitrile.

The (co)polymerization of the acrylonitrile may be carried out in a conventional manner, by radical solution polymerization, using a solvent of dimethylsulphoxide (DMSO) or dimethylformamide (DMF) type, optionally in the presence of an activator such as azobisisobutyronitrile (AIBN) or of an azocarboxylic acid ester, as described in the Patent Application US 2004/068069. As a variant, the (co)polymerization of the acrylonitrile may be carried out in aqueous dispersion in the presence of sodium thiocyanate, zinc chloride or sodium perchlorate, for example. Polymerization methods that can be used are in particular described in “Polymerization of acrylic fibers”, Encyclopedia of Polymer Science, Vol. 1, pp. 334-338, 1985. The homopolymer or copolymer obtained, denoted by PAN, in general has a weight-average molecular weight of around 80 000 to 120 000 g/mol.

This PAN is then formed into fibres in step (d) of the method according to the invention. This step may be carried out in several ways.

According to one method, the PAN is mixed with at least one plasticizer such as an alkyl carbonate, in particular ethylene carbonate (ester of ethylene glycol and of carbonic acid), it is heated at 110-160° C. in order to soften it and it is injected through a fine die, so that it falls into a bath, composed for example of water, where it coagulates and solidifies in the form of fibres. This method is quite similar to that used for manufacturing textile acrylic fibres.

According to another method, the PAN in dissolved in a solvent (DMSO, DMF, DMA or aqueous solution of inorganic salts) and it is injected through a fine die into a receiving chamber or drying oven where, after evaporation of the solvent, the polymer forms a solid fibre.

In all cases, the fibres thus obtained are washed and drawn until the desired fibre diameter is obtained. Drawing also makes it possible to align the molecular species, which will subsequently facilitate, during the carbonization, a correct formation of the carbon-carbon bonds and will provide the fibre with great solidity.

Before the actual carbonization, the fibres need to be modified chemically slightly in order to convert their atomic arrangement to a more crosslinked structure. This operation known as a stabilization or partial oxidation operation constitutes step (e) of the method according to the invention.

This operation is carried out by heating the PAN fibres at 200-300° C. for a few tens of minutes in the presence of air. In this manner, the fibre modifies its atomic arrangement and polar surface functions are created; it changes from a plastic state to a thermally stable infusible state. Since this reaction is exothermic, it is advisable to ensure that the heat transfers are controlled since thermal runaway could occur.

After the stabilization operation the actual carbonization, which constitutes step (f) of the method according to the invention, is carried out by heating the fibres from step (e) at a temperature between 1200 and 1500° C. in a furnace purged with an inert gas and maintained at a pressure above atmospheric pressure in order to prevent air from re-entering the furnace.

During the carbonization, most of the atoms, apart from the carbon, are expelled in the form of water vapour for oxygen and hydrogen, ammonia and hydrogen cyanide for nitrogen atoms, gaseous nitrogen, carbon monoxide and carbon dioxide originating from the polar surface functions. The expulsion of these atoms allows the carbon to be organized in microcrystalline form by creating strong bonds. The carbonization may optionally be carried out in two steps, at two different temperatures, for a better control of the entire process. The first carbonization step may thus be carried out at 400-800° C., the fibre optionally being stretched during this step.

At the end of this carbonization step, fibres known as “high strength” or “intermediate modulus” fibres are obtained, depending on the treatment temperature. A subsequent graphitization step between 2000 and 3000° C. optionally makes it possible to obtain fibres known as “high modulus” fibres.

After the carbonization/graphitization, other steps may be carried out, with a view to improving the contact of the fibre with the matrix into which it will be incorporated. It is thus possible to slightly oxidize its surface, either by treatment in the presence of air or carbon dioxide, or by treatment in the liquid phase with sodium hypochlorite, nitric acid or a solution of sulphuric acid, of sodium hydroxide and of ammonium bicarbonate, for example. All these operations must be well controlled to avoid the creation of surface defects which could cause defective adhesions to the matrices.

The fibres may also be subjected to a sizing or oiling treatment that aims to protect them during the transport, weaving or winding thereof. This treatment consists in applying a coating material to the fibres, this coating material being chosen to be compatible with adhesion agents used in the manufacture of the composites and which may, for example, be selected from epoxide resins, polyesters or polyurethanes.

Another subject of the present invention is the carbon fibres capable of being obtained according to the method described previously.

These carbon fibres are characterized in that they comprise a not insignificant amount of carbon of renewable or biobased origin or still contemporary origin, that is to say of ¹⁴C. Indeed, all carbon samples taken from living organisms, and in particular from the vegetable matter used in the first step of the method according to the invention, are a mixture of three isotopes: ¹²C, ¹³C and ¹⁴C in a ¹⁴C/¹²C ratio that is kept constant by continuous exchange of carbon with the environment and that is equal to 1.2×10⁻¹². Although ¹⁴C is radioactive and although its concentration therefore decreases over time, its half-life is 5730 years, so that the ¹⁴C content is considered to be constant from the extraction of the vegetable matter up to the manufacture of the fibres and even up to the end of their use.

More specifically, it is considered that the carbon fibres according to the invention have an isotopic ratio of their ¹⁴C content to their ¹²C content which is greater than 10⁻¹² but no more than 1.2×10⁻¹², where ¹⁴C represents the isotope having 6 protons and 8 neutrons whereas ¹²C represents the stable isotope having 6 protons and 6 neutrons. A carbon fibre containing 100% of renewable carbon contains at the most 1.2×10⁻¹² of ¹⁴C.

The 14C content of the carbon fibres may be measured according to well-known techniques for dating archaeological remains, old woods, bones, peat or even seashells. It may, for example, be measured according to the following techniques:

-   -   by liquid scintillation spectrometry: this method consists in         counting “beta” particles resulting from the disintegration of         ¹⁴C. The beta radiation resulting from a sample of known mass         (known number of carbon atoms) is measured over a certain time.         This “radioactivity” is proportional to the number of ¹⁴C atoms,         that it is thus possible to determine. The ¹⁴C present in the         sample emits beta radiation, which, in contact with the         scintillation liquid (scintillator) gives rise to photons. These         photons have different energies (between 0 and 156 keV) and form         what is known as a ¹⁴C spectrum. According to two variants of         this method, the analysis focuses either on the CO₂ previously         produced by the carbon-based sample in a suitable absorbent         solution, or on benzene after prior conversion of the         carbon-based sample to benzene.     -   by mass spectrometry: the sample is reduced to graphite or to         gaseous CO₂, then analysed in a mass spectrometer. This         technique uses an accelerator and a mass spectrometer to         separate the ¹⁴C ions from the ¹²C ions and therefore to         determine the ratio of the two isotopes.

These methods for measuring the ¹⁴C content of materials are described precisely in the ASTM D 6866 standards (especially D6866-06) and in the ASTMD 7026 standards (especially 7026-04). These methods measure the ¹⁴C/¹²C ratio of a sample and compare it to the ¹⁴C/¹²C ratio of a reference sample of 100% renewable origin, to give a relative percentage of carbon of renewable origin in the sample.

Another subject of the present invention is therefore carbon fibres that have an isotopic ratio of their ¹⁴C content to their ¹²C content which is greater than 2×10⁻¹¹, for example greater than 5×10⁻¹¹, and preferable greater than 10⁻¹² and at the most equal to 1.2×10⁻¹².

The carbon fibres according to the invention can be employed in all the applications where they are customarily used, especially for reinforcing composites, in particular in the manufacture of aircraft or spacecraft parts, sports equipment (tennis rackets and golf clubs) and wind turbines; in the filtration of high-temperature gases; as antistatic agents; as specialty electrodes; or as reinforcement for pressurized gas tanks, especially for storing hydrogen.

Another subject of the present invention is therefore these uses of the carbon fibres described previously. 

1. Method of manufacturing carbon fibres, comprising: a) synthesising acrolein from glycerol of vegetable origin; b) ammoxidizing the acrolein to obtain acrylonitrile; c) polymerizing the acrylonitrile to a homopolymer or copolymer of acrylonitrile (PAN); d) converting the PAN to PAN fibres; e) partially oxidizing the PAN fibres; and f) carbonizing the partially oxidized PAN fibres.
 2. Method according to claim 1, characterized in that the glycerol used in step (a) is obtained as a by-product of a transesterification of triglycerides of vegetable origin.
 3. Method according to claim 2, characterized in that the glycerol is obtained as a by-product in the manufacture of biodiesel from vegetable oil.
 4. Method according to claim 1, characterized in that the glycerol is dehydrated to acrolein, in step (a), by heating at a temperature ranging from 250 to 400° C.
 5. Method according to claim 1, characterized in that the acrylonitrile is copolymerized, in step (c), with at least one acrylic comonomer selected from the group consisting of methyl acrylate, methyl methacrylate and acrylic acid.
 6. Method according to claim 5, characterized in that the comonomer does not represent more than 10% by weight relative to the total weight of the monomers to be polymerized.
 7. Method according to claim 1, characterized in that, in step (e), is carried out by heating the fibres at 200-300° C. in the presence of air.
 8. Method according to claim 1, characterized in that, in step (f), is carried out by heating the fibres at a temperature between 1200 and 1500° C. in a furnace purged with an inert gas and maintained at a pressure above atmospheric pressure.
 9. Carbon fibres obtained following the method according to claim
 1. 10. Carbon fibres having an isotopic ratio of their ¹⁴C content to their ¹²C content which is greater than 5×10⁻¹¹.
 11. (canceled)
 12. Method according to claim 1, characterized in that the glycerol is dehydrated to acrolein, in step (a), by heating at a temperature ranging from 260 to 300° C.
 13. Carbon fibres having an isotopic ratio of their ¹⁴C content to their ¹²C co×ntent which is greater than 10⁻¹². 